1. Field of the Invention
The present invention relates to microwave ovens, and more particularly, to a structure of a microwave oven, which can cool down various components of the microwave oven, smoothly.
2. Background of the Related Art
In general, the microwave oven (MWO) cooks food with heat from friction between molecules caused by disturbance of the molecular arrangement of the food made with a microwave (approx. 2,450 MHz). The microwave oven may have one or more than one magnetrons.
The microwave oven with one magnetron is used as domestic use where the microwave oven is not used frequently, and the microwave oven with more than one magnetrons is used as commercial use for convenience store and the like where the magnetron is used frequently.
FIGS. 1˜5 illustrate inside structures of related art microwave ovens each with two magnetrons schematically, referring to which the microwave oven will be described.
Referring to FIG. 1, the related art commercial microwave oven is provided with an outer case 11, a base plate 12, an inner case 13 of a cooking chamber, a front panel 14, a rear panel 15, and an outfit room.
The front panel 14 is mounted in a front part of the base plate 12, and, as shown in FIG. 2, the base plate 12 has a plurality of inflow holes 12a for drawing external air.
The inner case 13 has a plurality of outflow holes 13a. The rear panel 15 is mounted in a rear part of the base plate 12, and, as shown in FIG. 3, has discharging holes 15a and 15b for discharging air.
Referring to FIGS. 4 and 5, in the outfit room, there are one pair of transformers 16a, and 16b, one pair of magnetrons 17a, and 17b, a fan 18a, and a fan motor 18b provided thereto.
The one pair of transformers 16a and 16b are mounted on the base plate 12 side by side. The one pair of the magnetron 17a and 17b are mounted in an upper part and a lower part of the inner case 13, respectively. A microwave from the magnetrons 17a and 17b propagates to an upper space and a lower space of the inner case 13 through guide ducts (not shown).
The fan 18a is mounted in a space between the transformers 16a and 16b, and the magnetrons 17a and 17b in the outer case 11, and connected to the fan motor 182b in a state protected with a fan housing 18c. 
The fan housing 18c has a suction side facing a lower space. The fan housing 18c is in communication with an air duct 19 for guiding air flow to the magnetrons 17a and 17b. An end of the air duct 19 is in communication with the discharging holes 15a and 15b in the rear panel 15.
A process for cooling the components of the microwave oven will be described in detail.
When the microwave oven is put into operation, the fan 18a rotates as the fan motor 18b is driven, to draw external air. The external air is introduced into the microwave oven through the inflow holes 12a in the base plate 12, and cools the one pair of transformers 16a and 16b as the external air moves toward the fan 18a. Then, the external air cools the magnetrons 17a and 17b through the air duct 19 in communication with the fan housing 18c. 
In this instance, a portion of the air passed through the magnetrons 17a and 17b is discharged through the discharging holes 15a in the upper part of the rear panel 15 via the outflow holes 13a. Rest of the air passed through the magnetrons 17a and 17b is discharged through the discharging hole 15b in a lower part of the rear panel 15.
However, the related art microwave oven has the following problems.
First, the one pair of magnetrons in the related art microwave oven, mounted in a rear space of the magnetron, can not but increase the rear space of the microwave oven. Therefore, the inner case becomes smaller in comparison of a total size of the microwave oven. Moreover, the increase size of the microwave oven to require a more installation space can not but limit an installation space.
Second, the position of the fan mounted in a corner of one side of the outer case impedes the external air introduced through the base plate to cool the front transformer, smoothly. That is, since the front transformer is mounted in a blind area of air flow, cooling of the transformer has not been smooth.
Third, the fan motor in the related art microwave oven generates much heat when driven. However, since the related art microwave oven is not provided with a separate structure for cooling the fan motor, overheat of the fan motor causes a poor performance.